1. Field of the Invention
The present invention relates to a method and radar for detecting a vehicle or the like by using, for example, radio waves in the millimeter-wave band.
2. Description of the Related Art
A known radar mounted on a vehicle such as an automobile for measuring, for example, the distance between automobiles is disclosed in Japanese Unexamined Patent Application Publication No. 2000-338222.
The radar disclosed in the above publication can direct beams in three directions by beam-scanning means. When a target object (hereinafter referred to as the target) is detected in a plurality of beam directions, the radar calculates the angle of the target in accordance with the reflected-signal intensities corresponding to the plurality of directions. When the target is detected in only a single direction, the radar determines that the angle of the target is the same as the predetermined angle.
However, the above-described radar is not able to track the target at a resolution higher than in a case where the target is detected within the width of a detection radio-wave beam. Since the angle corresponding to the highest reflected-signal intensity does not necessarily indicate the center of the target, the precision of finding the direction of the target is low.
Accordingly, it is an object of the present invention to provide a method for detecting and a radar with higher position-detecting precision. The method and radar are capable of detecting the direction of the target at a resolution higher than in a case where the target is detected within the width of the detection radio-wave beam. Further, the method and radar are capable of solving the problem that arises when the highest reflected-signal intensity does not correspond to the center of the target.
The radar comprises means for detecting reflected-signal intensities, each of the reflected-signal intensities obtained for a predetermined unit angle by changing the direction of a beam within a detection-angle range. The radar further comprises means for detecting data of a reflected-signal-intensity distribution by calculating moving averages of the reflected-signal intensities by using an angular width predetermined for a given distance as an average width.
Generally, when the width of the beam is larger than the unit angle predetermined for beam scanning (hereinafter referred to as the space between the beams), or when the target is larger than the space between the beams, the signals reflected from the target will be detected continually with the beam scanning. That is, the reflected signals are detected by neighboring plurality of beams.
However, the reflected-signal intensities greatly vary depending on the shape or direction of the target. Therefore, the intensity of the signal reflected from the center of the target, that is, the reflected-signal intensity of the center one of the above-described plurality of beams is not necessarily the highest. However, the reflective-signal intensities which change in accordance with the change of the angle of the beams form a peak-like shape, irrespective of the reflected-signal intensity of the each of beams. As described above, the moving averages of the reflected-signal intensities are calculated by using the predetermined angle width as the average width, for the given distance. Accordingly, the obtained data of the reflected-signal-intensity distribution, which shows the shape of the target projected by the beams, is unaffected by the direction of the target.
The radar further comprises means for detecting a direction of the highest reflected-signal intensity from among the data of the reflected-signal-intensity distribution, for the given distance. Accordingly, the direction corresponding to the center of the target is detected by simple processes.
The radar further comprises means for detecting a direction corresponding to the barycenter of the reflected-signal intensities from among the data of the reflected-signal-intensity distribution, for the given distance. Accordingly, the center direction of the target is more accurately detected by detecting the target within a range smaller than the space between the beams. That is, the target can be detected in a range narrower than the space between the beams. This means that the target can be detected without using the space between the beams as the unit of the detection.
The radar farther comprises means for detecting a direction corresponding to the center of an angular range where the reflected-signal intensities exceed a predetermined threshold value from among the data of the reflected-signal-intensity distribution, for the given distance. Suppose that signals are reflected from the target in a short distance, and are observed within a wide angular range. In such a case, the moving averages calculated from the reflected signals do not form a steep peak. However, the above-described means readily detects the center direction of the target. The above-described means removes reflected signals having low intensities or reflected signals which have been generated by something other than the target and reflected signals that are not important for detecting the center direction of the target, thereby easily detecting the center direction of the target.
In the present invention, the average width used for the moving-average processes is reduced, at least in the vicinity of both ends of the detection-angle range, with increasing proximity to both ends, whereby the moving-average processes can be performed even in the vicinity of both ends of the detection-angle range. Accordingly, the direction of the target can be detected within the entire detection-angle range.
Further, in this invention, the average width used for the moving-average processes is reduced as the distance between the radar and the target to be detected increases. This means that the moving average corresponds to the number of the beams which radiate the target. The number of the beams is determined according to the distance between an antenna and the target. Accordingly, a problem of the varying precision of finding the direction of the target, which arises from the changing distance between the antenna and the target changes, is solved.
In accordance with one aspect, the radar comprises a detection circuit for detecting reflected-signal intensities of reflected signals from a target, the detection circuit including an antenna, a circulator, a mixer and an IF amplifier, wherein the antenna receives the reflected signals and provides them to the circulator, the mixer receives the reflected signal from the circulator and mixes the reflected signal with a signal proportional to a transmitted signal to generate an IF signal, and the IF amplifier amplifies the IF signal, and wherein each of the reflected signal intensities is detected for a predetermined unit angle by changing the direction of a detection radio-wave beam within a detection-angle range; and a programmed CPU for performing moving-average processes for obtaining data of a reflected-signal-intensity distribution by calculating moving averages of the reflected-signal intensities by using an angular width predetermined for a given distance as an average width.
The method of determining the distance to a target comprises transmitting a detection radio-wave beam to the target; detecting reflected-signal intensities of signals reflected from the target, wherein each of the reflected signal intensities is detected for a predetermined unit angle by changing the direction of the detection radio-wave beam within a detection-angle range; and performing moving-average processes for obtaining data of a reflected-signal-intensity distribution by calculating moving averages of the reflected-signal intensities by using an angular width predetermined for a given distance as an average width.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.